This invention relates to a plural beam cathode ray tube and more particularly to a three-beam in-line bi-potential electron gun assembly as utilized in a color cathode ray tube.
Many of the cathode ray tubes presently utilized in color television display applications are of the type employing a patterned multi-phosphor cathodoluminescent screen interiorly disposed upon the viewing panel of the tube envelope wherein an apertured or multi-opening mask is spatially positioned in relation thereto. A plurality of electron beams, emanating from an electron gun assembly positioned within the neck portion of the tube envelope, are directed to converge at and traverse the apertured mask to thence selectively impinge and luminescently excite the electron responsive phosphors comprising the patterned screen therebeyond. Focusing of the individual electron beams is conventionally achieved by means of discrete electron lensing, such as bi-potential focus lensing; such being dependent upon the ratio of the focus voltage to the respective accelerating electrode or anode voltage.
The aforementioned cathodoluminescent screen is of the type made up of repetitive patterns formed, for example, of individual dots or stripes of red, green and blue-emitting phosphor components. Since these phosphor materials exhibit differences in efficiency, they require excitation by electron beams of different current levels to produce substantially equal light output. Additional differences in excitation current arise because of the non-uniform response of the human eye to various colors. Thus, to produce white light, more beam current is required to excite the green-emitting phosphor than is necessary to excite the respective red and blue associated color-emitting materials. Each of the exciting beams emanates from a separate electron gun structure which collectively constitute the gun assembly. In a conventional assembly the several cooperating electrode components of each gun are substantially dimensionally similar to the respective components of the related gun structures comprising the assembly.
The differences of operating intensities of the several electron beam producing guns functioning simultaneously within the tube to provide a desired white, are conventionally expressed in terms of at least two gun current ratios; namely, red to green (R/G) and red to blue (R/B). For example, in a tube having the red, green and blue electron guns operating in concert to provide a desired cathodoluminescent white, a red/green gun ratio of 1.4:1 indicates that an electron beam current of 40 percent greater intensity is required for the red gun than is needed for the green gun to provide the necessary individual brightness levels of the respective red and green emitting phosphors. Correspondingly, in the same tube, a red/blue gun ratio of 1.6:1 denotes that the red gun must deliver 60 percent more beam current than the blue gun to satisfactorily synthesize the white field in the simultaneously excited screen. In accordance with the electron-optics properties of electron guns evidencing substantially equal focusing lenses, the respective diameters of the several electron beams tend to "bloom" or become larger as the applied beam currents are increased. Thus, the apparent sharpness of the imagery evidenced in the screen of a color cathode ray tube is resolved in accordance with the respective beam diameters impinging the associated phosphor components of the patterned screen. Accordingly, reduced brightness and diminished resolution of imagery is evidenced with beam landings of larger spot size.